Rain Garden Size Calculator

How it works

Area = larger of [impervious area × soil factor (sand 20%, loam 30%, clay 45%)] and [runoff volume ÷ ponding depth ÷ 0.9]; runoff = (0.9 × impervious + 0.2 × lawn) × rainfall; drain time = depth ÷ infiltration rate

A rain garden is a shallow planted depression that catches roof and driveway runoff and lets it soak in over a day or so, instead of sending it down the storm drain with its oil, fertiliser and heat. Sizing follows the soil: on sandy ground a bed a fifth the size of the roof area draining into it is plenty, on clay nearly half — or the soil is replaced with a sand-compost mix over a gravel underdrain. The second check is volume: the bowl must hold the runoff from the design storm (25 mm captures the great majority of rain events in temperate climates), with a ponding depth of 10–20 cm so plants are not drowned. It must empty within 24–48 hours, sit at least three metres from foundations, and have an overflow for the big storms it cannot hold.

Worked example

80 m² of roof and 40 m² of lawn on silt loam with 15 cm ponding: a 25 mm storm yields 2,000 L; the soil rule gives 27 m² and the volume rule 14.8 m², so build about 27 m² (7.3 × 3.7 m), which drains in around 6 hours.

Frequently asked questions